Electrophotographic image forming apparatuses (for example, printers) include a fixing apparatus by which a toner image formed on printing paper is fused with heat and fixed thereon. As an example of such a fixing apparatus, there has been known a fixing apparatus of a roller pair system, in which a fixing roller and a pressure roller are provided, as described in Publications 1 to 4 below.
The fixing roller is a roller member with a hollow core of metal such as aluminum, whose surface is coated with an elastic layer and inside which is installed a halogen lamp as a heat source. The temperature of the fixing roller is controlled by a temperature control section, which controls ON/OFF of the halogen lamp based on a signal outputted from a temperature sensor provided on the surface of the fixing roller.
The halogen lamp is just one example of a means for heating the fixing roller. There has also been known an external heating member which is pressed against the surface of the fixing roller to heat the fixing roller. Specific examples of such an external heating member include an external heating roller disclosed in Publication 1, and external heating belts disclosed in Publications 2 through 4. Since the external heating member is directly in contact with the surface of the fixing roller, the external heating member can heat the surface of the fixing roller more quickly than the halogen lamp.
The pressure roller is a roller member with a metal core covered with a heat-resistant elastic layer, which is made of silicone rubber for example. The pressure roller is pressed against the surface of the fixing roller, which causes the pressure roller to undergo elastic deformation. As a result, a nip region is formed between the fixing roller and the pressure roller.
In the fixing apparatus having the foregoing arrangement, a printing paper with an unfixed toner image thereon is sent to the nip region between the fixing roller and the pressure roller, and the printing paper is transported by rotating the fixing roller and the pressure roller. The toner image on the printing paper is then fused with the transferred heat from the surface of the fixing roller. This fixes the toner image on the printing paper.
In such a fixing apparatus, offset phenomena such as cold offset and hot offset are known to occur when the surface temperature of the fixing roller falls outside of a suitable temperature range. The cold offset is a phenomenon in which some of the toner that did not melt property due to shortage of transferred heat to the printing paper adheres to the fixing roller. The hot offset is a phenomenon in which some of the toner on the printing paper adheres to the fixing roller due to weakened toner cohesive force caused by overheating of the toner on the printing paper.
In order to prevent cold offset and hot offset, it is very important in the fixing apparatus to control surface temperature of the fixing roller within a suitable temperature range during feeding of printing paper.
A suitable temperature range for the surface temperature of the fixing roller varies depending upon a printing paper transport speed of an image forming apparatus in which the fixing apparatus is installed. More specifically, a suitable temperature range tends to shift to higher temperatures with increase in transport speed (process speed) of printing paper, and to lower temperatures with decrease in transport speed of printing paper. The reason for this is that a fast transport speed affords only a short contact time for the printing paper and the surface of the fixing roller, and as such a relatively high surface temperature is required for the fixing roller in order to transfer a sufficient amount of heat to the printing paper. When the transport speed is slow, there is a long contact time for the printing paper and the surface of the fixing roller. In this case, the transfer of heat from the surface of the fixing roller to the printing paper will be in excess unless the surface temperature of the fixing roller is restrained.
A so-called four-cycle electrophotographic image forming apparatus is generally designed to have substantially the same printing paper transport speed for color and monochromatic images formed on printing paper. (It is noted however that the distance between transported sheets of printing paper is different between color mode and monochromatic mode, and as such the number of sheets processed per unit time is greater in the monochromatic mode.) A four-cycle electrophotographic image forming apparatus forms color images from overlaid toner images of four colors C, M, Y, K, which are formed by a set of visualized image forming units.
In a fixing apparatus for the four-cycle electrophotographic image forming apparatus, there is a sufficiently wide common non-offset range in the suitable temperature ranges (non-offset ranges) for fixing color images and monochromatic images, as shown in FIG. 9 (common non-offset range being an overlapping range of these non-offset ranges). It is therefore easy to set a control value (target value) for the surface temperature of the fixing roller within the common non-offset range, and control the surface temperature of the fixing roller within suitable temperature ranges in fixing both color and monochromatic images. In this manner, the offset phenomenon can easily be avoided in the four-cycle image forming apparatus.
Meanwhile, there is a new type of image forming apparatus employing a four-drum tandem engine, provided with four sets of visualized image forming units respectively corresponding to colors of C, M, Y, and K. In this type of image forming apparatus, there is an increasing demand for providing a faster printing paper transport speed for monochromatic images than for color images. In this design, there is a large difference in printing paper transport speed between a monochromatic mode forming monochromatic images and a color mode forming color images. The number of sheets processed per unit time in the monochromatic mode can also be increased in this manner, without causing deterioration of image quality in resulting monochromatic images.
However, in the image forming apparatus that employs such design to increase the number of processed sheets for monochromatic images, there is a significantly narrow common non-offset range in the non-offset ranges for fixing color and monochromatic images, as shown in FIG. 10. With such a narrow common non-offset range, it is difficult to control the surface temperature of the fixing roller within the common non-offset range even when a control value (target value) for the surface temperature of the fixing roller is set within the common non-offset range. This easily causes the problem of cold offset and hot offset.
Further, for feeding from standby, the surface temperature of the fixing roller needs to be set to enable feeding both in the monochromatic mode and the color mode. Thus, during standby, the surface temperature of the fixing roller needs to be controlled between a lower limit of fixing temperature for the monochromatic mode and an upper limit of fixing temperature for the color mode, and be ready for quick switching between the monochromatic mode and the color mode in subsequent feeding operations.
For accurate control of the surface temperature of the fixing roller in a fixing apparatus adapted for color images, an external heating member such as an external heating roller or an external heating belt that is brought into contact with the surface of the fixing roller is more suitable compared with an internal heating member such as a halogen lamp provided inside the fixing roller. In this respect, temperature control by the external heating member is important for controlling the surface temperature of the fixing roller within a narrow temperature range.
In one method of temperature control by the external heating member, the release/contact of the external heating member is controlled based on information indicative of surface temperature of the fixing roller, so as to instantly raise or lower the surface temperature of the fixing roller. The surface of the fixing roller can be controlled at a predetermined temperature by bringing the external heating member into contact with the fixing roller or separating it therefrom according to the print mode. Specifically, according to the print mode, the external heating member is brought into contact with the fixing roller to increase the surface temperature of the fixing roller, or separated therefrom to prevent overheating of the fixing member.
[Publication 1]
Japanese Unexamined Patent Publication No. 038802/1999 (Tokukaihei 11-038802; published on Feb. 12, 1999)
[Publication 2]
Japanese Unexamined Patent Publication No. 189427/2005 (Tokukai 2005-189427; published on Jul. 14, 2005)
[Publication 3]
Japanese Unexamined Patent Publication No. 292714/2005 (Tokukai 2005-292714; published on Oct. 20, 2005)
[Publication 4]
Japanese Unexamined Patent Publication No. 017031/1977 (Tokukaisho 52-017031; published on Feb. 8, 1977)